Insights into the tribochemistry of sliding iron oxide surfaces lubricated by sodium silicate glasses: An ab initio molecular dynamics study
© 2020 Elsevier B.V. Binary alkali silicate glasses are new promising lubricants for harsh working conditions, such as high pressure, elevated temperature, and high sliding speed. In the present work, the tribochemical reaction of sodium silicate under sliding contact with iron oxide substrates was investigated using First Principle Molecular Dynamics. The result indicates that the silicate is chemically active against the iron oxide surface at high temperature. Non-bridging oxygen (NBO) in glass lubricant and iron from the substrate forms the covalent bonds (NBO-Fe), which play a crucial role in the tribological performance. Interestingly, NBO in the glass can change continuously its covalent bond with different Fe atom of the substrate as a consequence of the shearing. Positively charged sodium atoms in the silicate network have an extremely high mobility, which allows them to diffuse towards the substrates, due to the electrostatic interaction with the negatively charged oxygen atoms of iron oxide substrates. These results explain the formation mechanism of a sodium-rich layer on iron oxide substrates. Reducing the sodium content in the glass lubricant will limit the formation of the sodium-rich tribofilm layer and increase the connectivity of the silicate network, which significantly reduce the friction and antiwear properties of the glass lubricant.